Scientists using NASA’s Hubble Space Telescope have discovered the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter’s largest moon. The subterranean ocean is thought to have more water than all the water on Earth’s surface.

Scientists estimate that Ganymede’s ocean is 60 miles (100 kilometers) thick – 10 times deeper than Earth’s oceans – and is buried under a 95-mile (150-kilometer) crust of mostly ice.

Jim Green is NASA’s director of planetary science. At a March 12 news teleconference Green said:

The solar system is now looking like a pretty soggy place.

Identifying liquid water is crucial in the search for habitable worlds beyond Earth and for the search for life, as we know it.

John Grunsfeld is the assistant administrator of NASA’s Science Mission Directorate. He said:

A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth.

Ganymede is the largest moon in our solar system and the only moon with its own magnetic field. The magnetic field causes aurorae, which are ribbons of glowing, hot electrified gas, in regions circling the north and south poles of the moon. Because Ganymede is close to Jupiter, it is also embedded in Jupiter’s magnetic field. When Jupiter’s magnetic field changes, the aurorae on Ganymede also change, “rocking” back and forth.

By watching the rocking motion of the two aurorae, scientists were able to determine that a large amount of saltwater exists beneath Ganymede’s crust, affecting its magnetic field.

This is an illustration of the interior of Jupiter’s largest moon Ganymede. It is based on theoretical models, in-situ observations by NASA’s Galileo orbiter, and Hubble Space Telescope observations of the moon’s magnetosphere, which allows for a probe of the moon’s interior. The cake-layering of the moon shows that ices and a saline ocean dominate the outer layers. A denser rock mantle lies deeper in the moon, and finally an iron core beneath that. Image credit: NASA, ESA, and A. Feild (STScI)

A team of scientists led by Joachim Saur of the University of Cologne in Germany came up with the idea of using Hubble to learn more about the inside of the moon. Saur said:

I was always brainstorming how we could use a telescope in other ways. Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior.

If a saltwater ocean were present, Jupiter’s magnetic field would create a secondary magnetic field in the ocean that would counter Jupiter’s field. This “magnetic friction” would suppress the rocking of the aurorae. This ocean fights Jupiter’s magnetic field so strongly that it reduces the rocking of the aurorae to 2 degrees, instead of 6 degrees if the ocean were not present.

Scientists first suspected an ocean in Ganymede in the 1970s, based on models of the large moon. NASA’s Galileo mission measured Ganymede’s magnetic field in 2002, providing the first evidence supporting those suspicions. The Galileo spacecraft took brief “snapshot” measurements of the magnetic field in 20-minute intervals, but its observations were too brief to distinctly catch the cyclical rocking of the ocean’s secondary magnetic field.

Bottom line: In March, 2015, NASA announced that the Hubble Space Telescope has the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter’s largest moon.